The mass spectrometry is a widespread measuring method for the analysis of ion masses that is distinguished by a high sensitivity, specificity, rapidity and economy. Therefore numerous applications of mass spectrometry are known in the basic research and the areas of analytic chemistry, medicine, pharmacy, semiconductor technology, environmental- and hydrocarbon research and characterization of nanomaterials. The mass spectrometry is based in general on a separation of ions as a function of their masses. The following three methods for the mass separation were previously known from the practice.
In the mass separation in a sector magnetic field an ion beam is conducted through a magnetic field in which the ions are guided on flight paths with different radii as a function of the mass-charge ratio. With the mass separation in the quadrupole filter ions are put in oscillation during the movement through a quadrupole ion conductor. The mass separation is based on the fact that certain ions for whose mass-charge ratio a resonance condition has been fulfilled in the ion conductor can pass the ion conductor and reach an ion detector. Finally, ions pass through a drift zone during the mass separation as a function of the flight time (TOF mass spectrometry) with a speed that is a function of the mass-charge ratio. With an ion detector, at first light and during the course of time heavier and heavier ions are detected.
All these conventional methods have the disadvantage that as a rule elaborate equipment with complex control and evaluation procedures are required for the mass separation. For example, for the mass separation in the sector magnetic field high voltages of around 10 kV are frequently used. The separation in the quadrupole filter requires an extremely precise adjustment of the field conditions in the ion conductor. Finally, a TOF mass spectrometer requires the implementation of a complicated time measuring technology. Due to the cited disadvantages the use of the conventional mass spectrometers is still limited. Robust, insensitive mass separation systems that can be routinely used are hardly available in the practice.
A mass spectrometer was described by W. H. Bennett in “Journal of Applied Physics” (vol. 21, 1950, p. 143 ff.) in which the mass separation takes place in an ion conductor with several grid electrodes arranged in series in the direction of movement of the ions. The grid electrodes are arranged in groups of three electrodes each, of which the middle electrode is loaded with a high frequency voltage. This arrangement of grid electrodes is permeable exclusively for ions with a certain mass so that they can be used as mass filter for the mass spectrometry. A disadvantage of this technology is that there is a fixed connection between the adjusted high frequency and the vertical distance of the electrode grids. It can be necessary, as a function of the mass of the ions to be detected, to change the distance between the electrode grids. A further disadvantage results from the fact that the ion conductor described by W. H. Bennett only has a limited mass dependency of the permeability so that the resolution power of the mass separation is limited.